1. Field of the Invention
The present invention relates generally to compositions and delivery systems for reducing turbulent drag associated with fluids flowing through conduits. In another aspect, the invention concerns low-viscosity drag reducing compositions which can be delivered to a remote location via a relatively small diameter injection line.
2. Description of the Prior Art
In the subsea production of oil and gas, production piping typically presents a significant bottleneck because of the difficulty and expense associated with the subsea installation of the piping. The production decrease caused by bottlenecking at subsea flowlines can have severe economic ramifications due to the resulting inability to run the hydrocarbon production system at full capacity. Several options that exist for preventing or curing bottlenecking at subsea flowlines include increasing the diameter of the flowlines, increasing the number of flowlines, or reducing the amount of friction loss in the flowlines to thereby allow more flow through the same diameter lines. The first two de-bottlenecking options of increasing the size or number of flowlines are obviously very expensive. Thus, it is highly desirable to be able to reduce friction losses in subsea flowlines.
It is commonly known that a variety of drag reducers are available for reducing the friction loss of a fluid being transported through a conduit in a turbulent flow regime. Ultra-high molecular weight polymers are known to function well as drag reducers; however, drag reducers vary in their effectiveness. Traditionally, the more effective drag reducing additives have been those containing higher molecular weight polymers. Increasing the molecular weight of the polymer generally increases the percent drag reduction obtained, with the limitation that the polymer must be capable of dissolving in the liquid in which friction loss is affected.
Many offshore oil and gas production facilities are operated from remote locations which can be miles away from the production wells. When remote facilities are used to operate a subsea production facility, an umbilical line is typically employed to provide power and various flow assurance chemicals to the production facility. Such umbilical lines generally include a plurality of relatively small-diameter injection lines through which various chemicals can be introduced into the flowline at an injection point proximate the production wells. These chemicals generally include low-viscosity fluids such as hydrate inhibitors, wax inhibitors, and corrosion inhibitors which help to improve flow conditions in the flowline.
In the past, it has been proposed that drag reducing agents could be transported through an umbilical line to thereby affect a reduction in friction loss in the subsea flowline. However, due to the high viscosity and/or large particle size associated with commercially available drag reducers, existing drag reducers cannot be transported through the relatively small diameter conduits an umbilical line without causing plugging or unacceptable pressure drop. U.S. Pat. No. 4,881,566 to Ubels et al (hereinafter, “the '566 patent”) discloses a method for transporting a high-viscosity, high-polymer-content drag reducer through a chemical injection line of an umbilical by facilitating the flow of the drag reducer with an immiscible low-viscosity liquid material injected at the periphery of the chemical injection line. However, the method described in the '566 patent requires special equipment for introducing the low-viscosity material into the periphery of the chemical injection line. Further, the '566 patent does not address the problems associated with drag reducers that require the formation of a strand (described below) to effectively dissolve in the host fluid.
Commercially available gel drag reducers are typically highly viscous (e.g., greater than 10,000 cP or sometimes greater than 100,000 cP at typical pumping shear rates) and highly concentrated with ultra-high molecular weight polymers. These conventional high-viscosity drag reducers must form a stable “strand” when injected into a pipeline in order for the polymer to properly dissolve. Strand formation is a viscoelastic property of a drag reducer that is dependent on many things, including the polymer molecular weight, the polymer concentration in the gel, the viscosity of the gel, and the injection dynamics. The presence of a strand provides sufficient surface area for the drag reducer to dissolve in a reasonable amount of time. Without sufficient strand stability, the drag reducer may not dissolve within the amount of time necessary to adequately reduce drag in the conduit. Strand stability can especially be a problem in multi-phase flow regimes, which are often present in offshore production lines. The high gas velocities and slugging associated with such multi-phase flow regimes result in highly chaotic environments that make stable strand formation nearly impossible. Thus, even if commercially available gel drag reducers could be effectively delivered to a remote production facility via an umbilical line, such gel drag reducers would still be ineffective due to their need to form a stable strand.
In the past, when drag reduction has been needed in chaotic flow environments that do not allow for stable strand formation, it has been necessary to utilize suspension or slurry drag reducers. However, conventional suspension or slurry drag reducers typically contain solid particulate matter that is too large to flow through an umbilical line without plugging the line.